CONDUCTION SYSTEM OF THE HEART (THE CARDIAC IMPULSE)

Dr. Ayisha Qureshi Professor

MBBS, MPhil

THE CONDUCTION SYSTEM OF THE

HEARTThe cardiac conduction system is a group of specialized cardiac muscle cells. These cells are capable of generating and conducting Action Potential. The conduction system of the heart comprises of: 1. The Sino-atrial node (SA

node). 2. The Atrio-ventricular node

(AV node). 3. The Bundle of His

(atrioventricular bundle). 4. Purkinje fibers.

Sino-Atrial Node(SA Node)

The sinus node (also called sinoatrial node) is a small, flattened, ellipsoid strip of specialized cardiac muscle, 3 X 15 X 1 mm.

It is located in the superior posterolateral wall of the right atrium immediately below and slightly lateral to the opening of the superior vena cava.

SA node has almost no contractile muscle filaments.

SA nodal fibres connect directly with the atrial muscle fibres so that any action potential that begins in the SA node spreads immediately to the adjacent atrial muscle.

SA node generates a Pacemaker potential.

Role of Internodal Pathways & the spread of

the AP to both Atria

Special bands are present b/w SA node & AV node and the velocity of conduction in these Inter nodal Pathways is about 1 m/sec. There are 3 Atrial Internodal Pathways: 1. Anterior Internodal Pathway or

Bachmann’s Bundle 2. Middle Internodal Pathway or

Wenkhebach Bundle 3. Posterior Internodal Pathway or Tract

of Thorel

If the AP can spread directly from the SA node to the atrial muscle why, is there the presence of Interatrial bands?Velocity of conduction in most atrial muscle is about 0.3 m/sec. These specialized bands conduct the AP at a speed of 1 m/sec (due to presence of specialized conduction fibers).

The atrial conduction system is organized so that the cardiac impulse does not travel from the atria into the ventricles too rapidly; there

is, in fact, a delay.

This delay allows time for the atria to empty their blood into the ventricles before

ventricular contraction begins.

The Atrioventricular Node (AV Node)

The A-V node is located in the posterior wall of the right

atrium immediately behind the tricuspid valve.

The Atrio Ventricular node continues into the AV bundle also called the Bundle of HIS.

The Impulse slows down at the AV node & the Bundle of HIS so

that there is a delay here before it enters into the

ventricles.

What is AV Nodal Delay?

It is the delay at the AV node to transmit the cardiac impulse forward. It is app. 0.09 second.

If delay at the AV Bundle (0.04 sec) is also included, then it equals 0.13 sec.

- What causes the AV Nodal Delay?

- Decreased number of gap junctions- Greater resistance to conduction of ions- Smaller fibres

WHAT CAUSES THE AV NODAL DELAY?

Why should the cardiac impulse travel from the atria into the ventricles after a certain

delay?

Because the delay will allow time for the atria to empty the blood into the ventricles & thus, complete ventricular filling, before ventricular

contraction begins.

How long does it take for the Cardiac Impulse to reach the ventricles?

How long does it take for the Cardiac Impulse to reach the ventricles?

• From SA node to the Av node: 0.03 sec• At the AV node: 0.09 sec• AV Bundle of HIS: 0.04 sec

• Time taken for the impulse to reach from the SA node to the ventricles: 0.16 sec

AV BUNDLE ALLOWS ONLY ONE-WAY CONDUCTION.

This is due to the fibrous skeleton of the heart.

The Bundle Branches and the Purkinje Fibres • Bundle of HIS extends from the AV node into the ventricles. • After about 10- 15 mm it divides into the Left and Right Bundle branches. • They move towards the apex of the heart. • Purkinje fibres emerge from the Bundle of HIS supply the ventricular muscle. • Purkinje fibres are very large fibres transmitting action potentials at a

velocity of 1.5 to 4 m/sec. The cause of the increased speed of conduction is: - Large sized fibres, - Increased no. of gap junctions, and- fewer myofibrils for contraction.

• Once impulse reaches the ends of Purkinje fibres (0.03 sec later), it is transmitted through the ventricular muscle mass by the ventricular muscle fibres themselves. (velocity of conduction: 0.3 to 0.5 m/sec)

• Impulse travels from the endocardium to the epicardium (0.03 sec)• Total time from Bundle of HIS to last of ventricular muscle fibre: 0.06 sec.

Purkinje Fibres ensure the synchronous contraction of the ventricular muscle.

Time taken for the impulse

• From SA node to the AV node: 0.03 sec• AV nodal delay: 0.13 sec

- 0.09 sec at AV node- 0.04 sec at Bundle of HIS

Thus, time taken for the impulse to reach from SA node to the ventricles: 0.16 sec• From Bundle branches to Purkinje fibers: 0.03 sec• From endocardium to the epicardium (after the

Purkinje fibers): 0.03 sec

Why does the SA node rather than the AV node or the Purkinje fibers

control the heart’s rhythmicity?

Why does SA node rather than the AV node or the Purkinje fibers

control the heart’s rhythmicity?

Because its rate of rhythmical discharge is faster than that of any

other part of the heart.

What is an Ectopic Pacemaker?

An Ectopic pacemaker of the heart is a pacemaker elsewhere than the

Sinus Node.

Question

Transmission of impulse from SA to AV node is blocked. Thus, SA node discharges at its own

rate while AV node becomes the new pacemaker and discharges at its own rate.

Why is this dangerous?

Question

What is Stokes Adams Syndrome?(Guyton, 13th Ed. Page: 127)